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Fire Debris

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  1. Fire Debris • Fire debris is submitted to laboratories for analysis by the fire marshal, crime scene investigators, forensic scientists, and insurance investigators. • Investigators determine the best locations at the scene to collect samples, based on suspicious details.

  2. Suspicious Circumstances • The presence of: • ignitable liquid (combustible) • accelerants (increase the rate of combustion) • These are frequently detected by canines trained to detect ignitable liquids by smelling the fire debris. • Investigators can also detect the presence of these liquids from pour patterns that remain on the burnt substrate after the fire.

  3. Pour Patterns • Pour patterns often are characterized by intermixed light, medium, and heavy burning in a puddle shape that corresponds to the shape of the original pool of the ignitable liquid.

  4. Suspicious Circumstances • Other indicators of ignitable liquid: • unnatural flame movement (downward or too fast) • gapping of wood or floor seams (caused by pooling of liquid) • damage with no identifiable point of origin • burned out flooring beneath appliances and furniture • Inverted cone shaped burn patterns on vertical surfaces

  5. Fire Debris Sampling • Sample areas likely to contain traces of ignitable liquid: • lowest regions of burned area • insulated areas within the pattern • porous substrates in contact with the pattern • cloth • paper products • wood • seams or cracks • lightly burned edges of the pattern

  6. Collecting Fire Debris • Once an appropriate area has been identified for sampling, samples are collected for later analysis. • Samples are collected in a tightly sealed container (glass jar or metal can).

  7. Collecting Fire Debris • Sample should fill 2/3 of the container. • Top 1/3 of container is used for sampling headspace.

  8. Passive Headspace Sampling • In passive headspace sampling the container is heated to volatilize any ignitable liquids remaining in the sample. • Activated charcoal is suspended in the headspace to absorb the volatilized liquid. • The charcoal is then removed from the sample container and the liquid is removed from it by solvent extraction.

  9. Solvent Extraction • Carbon disulfide is frequently used in solvent extraction because it produces excellent desorption of most accelerants. • It also produces a low detector response when analyzed by a gas chromatograph using a flame ionization detector. • Unfortunately, carbon disulfide is a health hazard because it can cause nervous system damage.

  10. GC-MS • After the solvent is extracted it is analyzed with a gas chromatograph – mass spectrometer (GC-MS). • The liquid is injected into the GC and carried through the instrument by an inert carrier gas (called the mobile phase). • The liquid then permeates a column (long thin tubing) which binds the liquid to a polymer coating on the inside (called the stationary phase). This separates the liquid components by volatility.

  11. GC-MS • More volatile components move faster through the column. • The components come off of the column separately during the elution step and enter the mass spectrometer. • The mass spectrometer separates each component based upon the mass-to-charge ratio of their particles. • The mass spectrum of the sample is then compared to a library of known compounds to identify the compound in the sample.

  12. Presumptive Testing • GC-MS analysis can be time consuming and expensive. • A presumptive test can be performed quickly in the field to indicate the presence of an ignitable liquid. • Colorimetric gas detection tubes are used for this purpose.

  13. Presumptive Testing • Colorimetric gas detection tubes are filled with a compound designed to react with a specific compound of interest (gasoline, diesel fuel, etc.) • To perform the test, both ends of the glass tube are broken off. • Air from the scene is drawn through the tube with a pump.

  14. Presumptive Testing • In the presence of specific vapors, the compound within the tube will change color. • The concentration of the compound may be estimated but this technique is not very accurate.

  15. Presumptive Testing • If the presumptive test is positive, then samples will be collected for GC-MS analysis. • In some labs, this is used for preliminary screening of samples.